Method for grinding

ABSTRACT

A method and apparatus for grinding for use with a grinding machine of the type comprising a circular grinding wheel adapted to rotate about its axis and a spindle adapted to hold a workpiece at a position radially spaced from the grinding wheel wherein the workpiece deflects away from the grinding wheel, and vice versa, as the grinding wheel is moved into the workpiece. The grinding machine includes means for advancing the grinding wheel into the workpiece, such as a threaded shaft and boss arrangement, and gauge means contact the workpiece on the spindle and provide an electrical output signal representative of the diameter of the workpiece. The advancing means is controlled by a microprocessor which, at the initiation of a grinding operation, rapidly advances the grinding wheel into contact with the workpiece a sufficient lineal amount to compensate for the total deflection of the workpiece, grinding wheel, and associated components. Thereafter the microprocessor advances the grinding wheel into the workpiece at a predetermined lineal rate. The output from the gauge means is coupled to the input of the microprocessor so that when the diameter of the workpiece attains predetermined dimension, the microprocessor controls the adavncing means to rapidly retract the grinding wheel from the workpiece at a third lineal rate. Both the first and the third lineal rates are greater than the mean second lineal rate.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to a method and apparatus forgrinding and, more particularly, to such a method and apparatus of thetype wherein a rotating grinding wheel is advanced toward a workpiecemounted on a spindle.

II. Description of the Prior Art

The previously known grinding machines typically comprise a circulargrinding wheel adapted to rotate about an axle and a spindle adapted tohold a workpiece at a position radially spaced from the grinding wheel.The spindle also rotates the workpiece about its axis.

The grinding wheel is mounted on a table which is radially movable sothat the grinding wheel can be moved into contact with the workpiece onthe spindle. Contact of the grinding wheel with the workpiece, ofcourse, initiates the grinding operation and removes material from theworkpiece in the conventional fashion. The rotation of the workpiece onthe spindle is necessary to insure that the material removed from theworkpiece is evenly distributed around the workpiece, thus, grinding atrue diameter of the workpiece.

Although any of a number of means may be utilized to advance the tablesupporting the grinding wheel toward the workpiece on the spindle,typically a threaded shaft cooperates with a threaded boss on the table.Consequently, rotation of the threaded shaft in a first rotationaldirection advances the grinding wheel toward the workpiece while,conversely, rotation of the shaft in the other direction retracts thegrinding wheel from the workpiece. Likewise, the speed of rotation ofthe shaft is directly proportional to the lineal or radial speed of thegrinding wheel toward or away from the workpiece.

In the previously known grinding machines, the grinding wheel isadvanced towards the workpiece either manually or at a constant andpredetermined lineal rate obtained from a steady state rotation of theshaft.

Conventional gauging means contact the outer periphery of the workpieceand provide an electrical output signal representative of the diameterof the workpiece. When the workpiece has been ground down to apredetermined diameter, the advancing means for moving the grindingwheel towards the workpiece are stopped.

Many previously known grinding operations typically comprise twoseparate grinding operations, namely a rough grinding operation and afinish grinding operation. In the rough grinding operation, the grindingwheel is advanced towards the workpiece at a given lineal speed, forexample, 0.004 inches per second which removes most of the excessmaterial from the workpiece. The finish grinding operation occurssubsequently and in this operation the grinding is also moved towardsthe workpiece in the above-described fashion but at a much slower rate,typically 0.0005 inches per second. A lesser amount of material isremoved from the workpiece during the finish grinding operation than inthe rough grinding operation but a smoother finish on the workpiece andmore accurate control of the workpiece diameter is obtainable during thefinish grinding operation.

With these previously known grinding methods and machines, as thegrinding wheel contacts and is moved into the workpiece on the spindle,the workpiece bends or deflects away from the grinding wheel and viceversa from the force exerted on the workpiece by the grinding wheel. Theamount of total deflection of the workpiece, the grinding wheel, and theassociated components, of course, depends upon the grinding machine andthe material and construction of the workpiece. However, for apredetermined grinding machine and type of workpiece, the amount oftotal deflection for a given grinding feed rate remains fairly constant.

During a grinding operation, as the grinding wheel moves initiallytowards and into the workpiece, a relatively small quantity of materialis initially removed from the workpiece due to the aforementioned totaldeflection despite a constant advance of the grinding wheel into theworkpiece. Only when the workpiece, the grinding wheel, and theassociated components are fully deflected will the grinding wheel removematerial from the workpiece at the rate of advance of the grindingwheel.

This previously known grinding method is disadvantageous in that only arelatively small amount of material is initially removed from theworkpiece as the grinding wheel contacts the workpiece. Since the rateof advance of the grinding wheel towards the workpiece is relativelyslow, this phenomena unnecessarily prolongs the time required for asingle grinding operation. For example, a typical grinding operationrequires approximately eleven seconds.

A still further disadvantage of these previously known grinding methodsis that due to the total deflection, even when the advance towards theworkpiece is halted, the workpiece and grinding components continue tostraighten out, i.e., to move toward an undeflected condition. This, ofcourse, results in the continued removal of material from the workpieceand often results in removing more material from the workpiece thantolerances permit.

A still further disadvantage of these previously known grinding machinesand methods is that due to the prolonged grinding time required by thepreviously known machines, the workpiece becomes heated by the frictionand expands accordingly; consequently, when the workpiece cools afterthe grinding operation, the shrinkage incurred during the cooling oftenrenders the workpiece smaller than the tolerance levels permit so thatthe workpiece must be scrapped.

The previously known grinding machines and methods, thus, suffer twoprimary disadvantages. First, such machines and methods are timeconsuming in operation and second, these prior machines and methods forma large percentage of improperly formed workpieces. Improperly formedworkpieces are scrapped.

SUMMARY OF THE PRESENT INVENTION

The present invention overcomes the above-mentioned disadvantages byproviding a method for grinding which both rapidly and accurately grindsa workpiece.

In brief, with the method of the present invention the grinding wheel isfirst rapidly advanced toward the workpiece a predetermined linealamount sufficient to compensate for the total deflection of theworkpiece, the grinding wheel, and the associated grinding components.

Thereafter the grinding wheel is moved linearly into the workpiece at apredetermined grinding rate until the workpiece obtains a predetermineddiameter or size. At that time, the grinding wheel is rapidly retractedfrom the workpiece thereby terminating the grinding operation.

As will become hereinafter more clearly apparent, by rapidly moving thegrinding wheel into the workpiece a lineal amount sufficient tocompensate for the total deflection at the initiation of the grindingoperation, the grinding wheel more rapidly and efficiently removesmaterial from the workpiece than the previously known grinding methods.Moreover, by rapidly retracting the grinding wheel out of contact withthe workpiece at the end of a grinding operation, machining tolerancescan be closely maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention will be had uponreference to the following detailed description when read in conjunctionwith the accompanying drawing, wherein like reference characters referto like parts throughout the several views, and in which:

FIG. 1 is a plan diagrammatic view showing a grinding machine accordingto the present invention;

FIG. 2 is a fragmentary top plan view showing the grinding machineaccording to the present invention with parts removed and enlarged forclarity;

FIG. 3 is a graph illustrating a prior art grinding method;

FIG. 4 is a block diagrammatic view showing the components of thegrinding machine of the present invention;

FIG. 5 is a flow chart illustrating the steps of the grinding method ofthe present invention;

FIG. 6 is a chart illustrating the grinding method and apparatus of thepresent invention; and

FIG. 7 is a graph similar to FIG. 3 but according to the method andapparatus of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

With reference to FIGS. 1 and 2 the grinding device 10 of the presentinvention is thereshown. The grinding device 10 includes a supporthousing 12 upon which a workpiece spindle 14 is secured. The workpiecespindle 14 is adapted to carry a workpiece 16 in the conventional mannerand conventional means 18 rotatably turn the workpiece 16 via thespindle 14 at any desired rotational speed.

A circular grinding wheel 20 is rotatably mounted on an axle 21 which inturn is carried by supports 22 so that the axis of rotation 24 of thegrinding wheel 20 is substantially parallel to the axis of rotation ofthe workpiece 16.

The grinding wheel support 22 in turn is secured to a carriage 26 whichis slidably mounted on appropriate tracks 28 secured to the supporthousing 12. Conventionally a hydraulic cylinder and piston arrangement30 is operatively connected to the carriage 26 and is adapted to shiftthe carriage 26 linearly toward or away from the workpiece 16. Thehydraulic cylinder and piston arrangement 30 typically moves thegrinding wheel 20 from a position spaced from the workpiece (in order toprovide easy access to the workpiece) to a position adjacent but nottouching the workpiece 16. The hydraulic arrangement 30 is inoperativeduring a grinding operation and, thus, will not be further described.

In order to linearly move the grinding wheel 20 toward the workpiece 16at a slow and steady rate, as would occur during a grinding operation,typically a threaded shaft 32 is axially constrained to the supporthousing 12 and threadably engages a threaded boss 34 in the carriage 26.Consequently, rotation of the shaft 32 in one rotational directionadvances the grinding wheel 20 towards the workpiece 16 while,conversely, rotation in the other direction retracts the grinding wheel20 from the workpiece 16. As should also be apparent, the speed ofrotation of the shaft 32 is directly proportional to the lineal speed ofthe carriage 16, and, hence, the lineal speed of the grinding wheel 20.Other means, of course, can also be utilized to advance the grindingwheel into the workpiece.

It will be understood, however, that alternatively the grinding wheel 20and axle 21 can be rotatably constrained to the support housing 12 whilethe workpiece spindle 14 is carried by the carriage 26. In this eventthe workpiece 16 is moved into the grinding wheel 20 during a grindingoperation.

With reference now particularly to FIG. 2, as the grinding wheel 20 ismoved linearly towards the workpiece 16, the workpiece 16 deflects apredetermined amount in dependence upon the material and size of theworkpiece 16. This deflection is shown with great exaggeration inphantom line in FIG. 2 and while the precise amount of deflection mayvary between different workpieces, in practice, workpieces constructedof substantially the same material and of substantially the same sizewill exhibit substantially the same amount of deflection during agrinding operation.

Similarly, the components of the grinding machine 10 bend or deflectaway from the workpiece 16 as the grinding wheel 20 contacts theworkpiece 16. These components of the grinding machine 10 which bendinclude, for example, the grinding wheel axle 21, the carriage 26, theshaft 32, the spindle 14, and the base of the machine 10. The deflectionof the grinding wheel axle 21 is shown, with great exaggeration, inphantom line. This deflection also remains fairly constant for a givengrinding feed rate.

The sum lineal deflection of both the workpiece 16 and the components ofthe grinding machine 10, hereinafter called the total deflection D forsimplicity, remains substantially constant for a particular rate oflineal advance of the grinding wheel 20 into the workpiece 16.

The grinding machine 10 that has been thus far described is ofconventional construction and is commercially available. With referencenow to FIG. 3, a graph is thereshown with Grinding Wheel Horsepowerindicia on its vertical axis and Time on its horizontal axis. During agrinding operation, the grinding wheel horsepower increases from arelatively low value and stabilizes at a relatively high value independence upon the grinding feed rate. During stabilization, the rateof material removal from the workpiece 16 is substantially constant.

FIG. 3 illustrates the prior art grinding method in which the grindingwheel 20 is linearly advanced towards the workpiece 16 at a steady rate,such as 0.004 inches per second. With reference now to FIG. 3 thegrinding wheel 20 contacts the workpiece at time T2 and, consequently,the grinding wheel horsepower 130 increases.

Between time T2 and T4, the workpiece 16 deflects or bends away from thegrinding wheel 20 and vice versa so that the rate of material removalfrom the workpiece 16 increases from time T2 to time T4. The workpiecediameter 132, thus, gradually decreases between T2 and T4.

At time T4 the grinding wheel 20 has advanced into the workpiece 16 alineal distance sufficient to compensate for the total deflection D sothat the rate of material removal from the workpiece is thereafterconstant and dependent upon the lineal feed rate or rate of advancementof the grinding wheel 20. Likewise the workpiece diameter 132 decreaseslinearly after time T4.

At time T5 the rotation of the shaft 32, and hence the advance of thegrinding wheel 20 towards the workpiece 16, is halted. At this time,however, the workpiece is in a deflected condition and straightens outagainst the rotating grinding wheel 20 and vice versa. Consequently,between times T5 and T7 the grinding wheel 20 continues to removematerial from the workpiece 16 as the workpiece 16 bends back into thegrinding wheel 20 and vice versa. The rate of material removal from theworkpiece 16 decreases from time T5 to time T7 so that the workpiecediameter 132 gradually becomes constant.

Typically, the rough grinding operation illustrated in FIG. 3 from T2 toT7 is immediately followed by a finish grinding operation shown from T8to T9. In a finished grinding operation, the rate of advance of thegrinding wheel 20 towards the workpiece 16 is considerably slower thanthe rough grinding operation illustrated in FIG. 3. However, the finishgrinding operation is illustrated in FIG. 3 like the rough grindingoperation but in smaller proportion.

In a manner which will hereinafter be described in greater detail,according to the present invention, the grinding wheel 20 is first movedat a high mean lineal rate into the workpiece 16 a predetermined linealdistance which is substantially the same as the total deflection D.Thereafter the grinding wheel 20 is advanced into the workpiece 16 at aconstant grinding rate and, when the workpiece 16 is ground to apredetermined diameter, the grinding wheel 20 is rapidly retracted awayfrom the workpiece 16 at a high mean lineal speed rate. The net effectof the present invention, then, is to greatly reduce the time between T2and T4 and also between T5 and T7 thereby greatly reducing the overallrequired grinding time while increasing accuracy.

The means for controlling the position of the grinding wheel 20 relativeto the workpiece 16 is illustrated diagrammatically in FIG. 4. Withreference to FIG. 4 a caliper gauge 34 having feeler arms 38 engages theworkpiece 16 and generates an electrical signal representative of thediameter of the workpiece 16 along line 40. Such gauges are well knownin the grinding field and will, therefore, not be described in greaterdetail.

The output line 40 from the gauge 36 is fed through an analog/digital(A/D) converter 42 to appropriate input lines of a microprocessor 44 sothat the precise diameter of the workpiece 16 is available to themicroprocessor 44 at any given moment of time.

The microprocessor 44, in response to the input signal from the gauge 36and according to a set of preprogrammed instructions contained withinthe microprocessor and which will subsequently be described in greaterdetail, generates output signals which control a high speed steppingmotor 50 in a manner to be shortly described. The stepping motor 50 ismechanically coupled to the shaft 32 (see FIG. 1) so that rotation ofthe stepping motor 50 rotatably drives the shaft 32.

Still referring to FIG. 4, the microprocessor 44 generates a digitaloutput along line 52 which is fed through a digital to analog (D/A)converter 54 having its output connected to an analog to frequency (A/F)converter 56 (such as a voltage controlled oscillator) so that thefrequency output on line 58 from the A/F converter 56 is proportional tothe digital output on line 52 from the microprocessor 44. The line 58 inturn is coupled to a conventional step motor drive 60 which generates anoutput along line 62 to the stepping motor 50 to control the rotationalspeed of the motor 50.

A separate output line 64 from the microprocessor 44 is coupled to thedrive 60 and controls the direction of rotation of the stepping motor 50through the drive 60. Consequently, with this arrangement the outputs 52and 64 from the microprocessor 44 control both the speed and directionof rotation of the stepping motor 50 and, hence, the speed and directionof the carriage 26.

The output line 58 from the A/F converter 56 is also preferably coupledto the clock input of an up/down counter 66 while the direction line 64from the microprocessor 44 is coupled to the control input of theup/down counter 66. The count in the counter 66 is representative ofrotational position of the stepping motor 50 and, thus, of the linealposition of the grinding wheel.

An error counter 70 is provided to detect an error or difference betweenthe number of pulses generated by the A/F converter 56 and the number ofpulses received by the stepping motor 50. The error counter 70 ispreferably an up/down counter and is coupled by lines 86 and 88 to A/Foutput line 58 and direction line 64, respectively, so that with thedirection line 64 at one state, the counter 70 is incremented by everypulse from the A/F output line 58. Similarly, the stepping motor 50generates a pulse to the counter 70 along line 72 for each pulsereceived by it from the drive 60 and an output on line 74 to the counter70 representative of the direction of rotation of the motor 50. Thepulses on line 72, however, decrement the counter 70 when the signal onthe direction line 72 corresponds to the last mentioned state on thedirection line 64 so that the counter 70 is sequentially incremented anddecremented (or vice versa if the direction lines 64 and 72 areinverted) for every pulse generated by the A/F converter 56 and receivedby the stepping motor 50. The count in the counter 70 thus representsthe total error between the signal transmitted to the motor 50 and thesignal received by the motor 50. An error greater than a selectableamount is detected by a detector 76 which generates an error signalalong line 78 to the microprocessor 44.

With reference now to FIGS. 5 and 6, a grinding operation according tothe present invention is there illustrated in flow chart and graph form,respectively. As should be apparent, the flow chart in FIG. 5 isrepresentative of a set of preprogrammed instructions contained withinor available to the microprocessor.

At step 100, which corresponds to T1-T2 in FIG. 6, the gap between thegrinding wheel 20 and the workpiece 16 is eliminated so that thegrinding wheel 20 is adjacent to but not touching the workpiece 16. Themicroprocessor 44 correlates the output from the caliper gauge 36 andthe input from the counter 66 to compute the lineal distance (and,hence, the time duration between T1 and T2) necessary to move thegrinding wheel 20 adjacent to the workpiece 16.

At step 102, the microprocessor 44 generates output signals along lines52 and 64 to rapidly advance the grinding wheel 20 into the workpiece 16substantially the lineal distance sufficient to compensate for the totaldeflection D. Step 102 is illustrated between times T2 and T4 in FIG. 6.Ideally, the grinding wheel 20 is instantaneously moved into theworkpiece 16 the lineal distance necessary to compensate for the totaldeflection D. However, due to mechanical limitations and inertia, thegrinding wheel 20 is rapidly accelerated linearly into the workpiece 16between times T2 and T3 and rapidly decelerated between times T3 and T4so that the mean linear velocity between times T2 and T4 is indicated inphantom line at 80.

At step 104, the grinding wheel 20 is advanced at the desired grindingrate lineally into the workpiece 16. Step 104 is illustrated betweentimes T4 and T5 in FIG. 6. Although the lineal rate of advancement ofthe grinding wheel 20 between times T4 and T5 is shown as constant, itmay be variable while remaining within the scope of the invention. Forexample, lineal rates of 0.004 and 0.0005 inches per second are typicalfor a rough and finish grinding operation, respectively.

At time T5, as determined at step 106, the diameter of the workpiece 16attains a predetermined size "A" which is sensed by the gauge 36 and fedinto the microprocessor 44.

At time T5 step 108 in the microprocessor 44 produces output signals onlines 52 and 64 to rapidly retract the grinding wheel 20 away from theworkpiece 16 to a position out of contact with the workpiece 16.Ideally, at time T5 the grinding wheel 20 is instantaneously retractedout of contact with the workpiece 16 but, due to the aforementionedmechanical limitations, the grinding wheel 20 is first rapidlyaccelerated away from the workpiece 16 between T5 and T6 and thendecelerated between T6 and T7 until the lineal movement of the grindingwheel 20 is stopped.

Typically following T7, a finish grinding cycle between T8 and T9 iseffected. The finish grinding cycle is substantially the same as therough grinding cycle that has been heretofore described in detail exceptthat the grinding rate determined at step 104 is usually much slowerthan for the rough grinding operation. Thus, for the sake of brevity,the finish grinding operation will not be described in detail.

With reference now to FIG. 7, a grinding operation according to thepresent invention is thereshown in graph form which corresponds to theprior art graph shown in FIG. 3. It should also be apparent that thetimes T1-T9 in FIGS. 3, 7, and 6 all correspond to each other.

Due to the rapid advancement of the grinding wheel 20 into the workpiece16 between T2 and T4, the elapsed time between T2 and T4 is much shorterthan the corresponding time duration taught by the prior art.Consequently, the workpiece diameter 110 decreases almost linearlybetween T2 and T7. The gradual reduction of the workpiece diameter 110between T7 and T8 in FIG. 7 represents thermal contraction as theworkpiece 16 cools. Between times T8 and T9, i.e., during the finishgrinding operation, the workpiece diameter also decreases virtuallylinearly as shown in FIG. 7 but at a more gradual slope than for therough grinding operation.

The grinding method and apparatus of the present invention thus achievesmany advantages over the prior art methods. Most notably, the totalgrinding time required is much less than the prior art methods whichprovides more efficient use of the grinding machine. For example, byrapidly advancing the grinding wheel into the workpiece at the start ofthe grinding operation and rapidly retracting the grinding wheel awayfrom the workpiece at the end of the grinding operation, the elapsedtime T2-T4 and T5-T7 (see FIGS. 3 and 7) is much less with the presentinvention than the corresponding prior art time periods. The finishgrinding operation, of course, can immediately follow the rough grindingoperation.

In addition, with a shorter grinding time the thermal expansion of theworkpiece is less than with the prior methods so that more accuratedimensional control of the workpiece is obtainable.

Having thus described our invention many modifications thereto willbecome apparent to those skilled in the art to which it pertains withoutdeviating from the spirit of the invention as defined by the scope ofthe appended claims. For example, while it is preferred to use themicroprocessor 44 to control the actuation of the stepping motor 50,alternatively, the control circuits for the stopping motor 50 can behard wired. The microprocessor 44, however, is preferred due to itsflexibility obtainable from its reprogrammability. For example thegrinding rate determined at step 104 (FIG. 5) can be changed by merelyreprogramming the microprocessor.

We claim:
 1. A method for grinding for use in conjunction with agrinding machine of the type comprising a first and second member,wherein one member is a circular grinding wheel adapted to rotate aboutits axis and the other member is a spindle adapted to hold a workpieceat a position linearly spaced from said grinding wheel, wherein onemember is linearly movable relative to the other member and whereby saidworkpiece and the components of the grinding machine bend apredetermined amount as said grinding wheel is moved into said workpieceat a constant predetermined lineal rate, the improvement comprising thesteps of:deflecting said workpiece and said components saidpredetermined amount by contacting and rapidly advancing the movablemember into the other member for said predetermined amount; immediatelythereafter advancing the movable member at said constant predeterminedlineal rate toward the other member whereby said workpiece and saidcomponents of said grinding machine remain deflected said predeterminedamount; and thereafter rapidly retracting the movable member away fromthe other member so that the grinding wheel is moved out of contact withsaid workpiece when the diameter of said workpiece attains apredetermined size.
 2. The method as defined in claim 1 wherein saidfirst step further comprises the steps of:accelerating the movablemember toward the other member; and thereafter decelerating the movablemember whereby said grinding wheel has substantially advanced into saidworkpiece said predetermined amount at the end of said deceleration. 3.The method as defined in claim 1 wherein said last step furthercomprises:accelerating the movable member away from the other member;and thereafter decelerating the movable member whereby said grindingwheel is out of contact with said workpiece at the end of saiddeceleration.
 4. The method as defined in claim 1 wherein said workpieceand the components of the grinding machine bend a second predeterminedamount as said grinding wheel is moved into said workpiece at a secondconstant predetermined lineal rate, the improvement further comprisingthe steps of:deflecting said workpiece and said components said secondpredetermined amount by contacting and rapidly advancing the movablemember into the other member for said second predetermined amount;immediately thereafter advancing the movable member at said secondconstant predetermined lineal rate toward the other member whereby saidworkpiece and said components of said grinding machine remain deflectedsaid second predetermined amount and wherein said second constantpredetermined lineal rate is less than said first constant predeterminedlineal rate; and thereafter rapidly retracting the movable member awayfrom the other member so that the grinding wheel is moved out of contactwith said workpiece when the diameter of said workpiece attains a secondpredetermined size.